As with all research, the aim is to isolate variables, which can then be analyzed and compared to other parameters, without the effect of extraneous variables. This study investigated energy contribution from fat while maintaining exercise intensity at a fixed level. So actually, the type of exercise and intensity, are unimportant.
What this research shows is that maximum speed at which fat can supply the body with energy in times of famine (hypophagia). Every process has its upper speed limit, and that speed limit was the aim of this study.
The problem with this study, and with all of science, is that the results need not apply to other situations, with altered variables. If the participants would instead have engaged in intense exercise, or have been sedentary, then the speed limit of fat energy supply could be different. So a discussion of the type of exercise is not needed, but further studies have to be done with altered variables. What is interesting, however, is what you mentioned about body fat percentage. They did not discuss the effect that fat percentage has on the fat energy supply. They only kind of titrated the caloric deficit to test the average speed of fat utilization in a population. To follow up on this, they should do the same thing, but for people of different fat percentages. Two possibilities arise: 1) either the maximum speed of fat breakdown is regulated extrinsically: through enzymes. Then this would be less dependent on body fat percentage. Or 2) it is regulated intrinsically: determined by the volume of fat. Or more specifically, the surface area of fat that is in contact with blood vessels. If this is the case, then body fat percentage would have a direct correlation to the maximal speed limit of energy utilization of fat.
Also, to answer your question about weight lifting and muscle sparing. You cannot measure the energy contribution by fat directly. You can only measure it indirectly. I'm assuming they quantitatively measured fat metabolites in the blood of the participants, and by that calculated the energy supply. What they say in the study is that if you eat slightly less, then you will have a caloric deficit that will be covered through you fat stores. If you eat much less, then your fat stores can't cover the energy demands, and your body starts breaking down muscle tissue. Im assuming they did not do weight lifting during the study, as that would have added an unneccesary variable (since it would have antagonized the muscle wasting in greater hypophagia); first you have to examine how the body functions in isolation, it is only later that you can alter variables and check how they affect results for for example athletes performing weight lifting, or sedentary people eating junk food.
I hope I cleared your confusion at least slightly; I'm sorry if I didn't answer exactly what you were wondering about, but from what I understand, you wanted an explanation of the design and results of the study. If there is something else or if something wasn't adequately explained, then comment and I will try a different approach.
